Multi-variable predictive controller

ABSTRACT

A multi-variable predictive controller is used to separate a multi-phase fluid on an offshore platform, thereby reducing condensable material in the gas product stream. A separation vessel containing a multi-phase fluid is provided, and pressure and temperature associated with the separation vessel are monitored. A Reid Vapor Pressure is calculated for the separation vessel based on the pressure and the temperature associated with the separation vessel. The multi-variable predictive controller actively controls the pressure and the temperature associated with the separation vessel such that the calculated Reid Vapor Pressure for the separation vessel is maintained within a predetermined amount of a reference Reid Vapor Pressure.

CROSS-REFERENCE TO A RELATED APPLICATION

The present application for patent claims the benefit of United Statesprovisional patent application bearing Ser. No. 61/716,357, filed onOct. 19, 2012, which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a multi-variable predictive controller,and more specifically, to a multi-variable predictive controller used toreduce condensable material in a gas product stream.

BACKGROUND

Production from offshore platforms, or rigs, generally includes acombination of crude oil and natural gas. Primary separation typicallytakes place on the offshore platform such that the wet crude oil istransported to shore via a first pipeline and the produced natural gasis dehydrated, compressed and transported to shore via a secondpipeline. As used herein, the term offshore platform comprises anyplatform structure, affixed temporarily or permanently to offshoresubmerged lands, that houses equipment to extract hydrocarbons from theocean or lake floor and that processes and/or transfers suchhydrocarbons to storage, transport vessels, or onshore. In addition,offshore production can include secondary platform structures, storagetanks associated with the platform structure, and floating productionand storage offloading equipment (FPSO).

Once separated from the wet crude oil, the produced natural gas beingexported from the offshore platform in the gas product stream typicallycontains amounts of hydrocarbon components in a liquid state, which arereferred to herein as hydrocarbon condensate. The hydrocarboncondensate, which comprises butanes, pentanes, and heavier molecules,increase the British Thermal Unit (BTU) value of the gas entering theoffshore facility's compressors, thereby reducing the efficiency of thecompressors. Moreover, additional equipment is needed at the onshore gasprocessing plant for separation and stabilization to extract condensatefrom the gas product stream. Accordingly, a better method is needed toreduce condensable material in the gas product stream prior to beingexported from the offshore platform.

SUMMARY

Embodiments disclosed herein relate to a method for separating amulti-phase fluid on an offshore platform. A separation vesselcontaining a multi-phase fluid is provided. A pressure and a temperatureassociated with the separation vessel are measured. A Reid VaporPressure for the separation vessel is calculated based on the pressureand the temperature associated with the separation vessel. The pressureand the temperature associated with the separation vessel are accuratelycontrolled such that the calculated Reid Vapor Pressure for theseparation vessel is maintained within a predetermined amount of areference Reid Vapor Pressure.

Embodiments disclosed herein relate to a system for separating amulti-phase fluid on an offshore platform. The system includes aseparation vessel, a pressure sensor, a temperature sensor, and amulti-variable predictive controller. The separation vessel receives amulti-phase fluid (e.g., produced fluids). The pressure sensor monitorspressure associated with the separation vessel and the temperaturesensor that monitors temperature associated with the separation vessel.The multi-variable predictive controller actively controls the pressureand the temperature associated with the separation vessel such that acalculated Reid Vapor Pressure for the separation vessel is maintainedwithin a predetermined amount of a reference Reid Vapor Pressure.

In embodiments, the pressure and the temperature associated with theseparation vessel are adjusted simultaneously based on the calculatedReid Vapor Pressure. The Reid Vapor Pressure can be calculated accordingto the thermodynamics of butane. For example, the Reid Vapor Pressurecan be calculated according to the following equations:

RVP=10^([A−B/(100+C)])+Bias

A=Log P ₁ +B/(T₁ +C)

B=(Log P ₁−Log P _(ref))/[1/(T _(ref) +C)−1/(T ₁ +C)]

In embodiments, the calculated Reid Vapor Pressure for the separationvessel is maintained within ten percent of the reference Reid VaporPressure. In other embodiments, the calculated Reid Vapor Pressure forthe separation vessel is maintained within five percent of the referenceReid Vapor Pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a multi-variable predictive controller,according to an embodiment of the present invention.

FIG. 2 is a schematic of a multi-variable predictive controller,according to an embodiment of the present invention.

FIG. 3 illustrates a graph of Reid Vapor Pressure using a regulatorySingle Input Single Output (SISO) control whose set points are given bya human operator and of Reid Vapor Pressure controlled using anautomated multi-variable predictive controller.

DETAILED DESCRIPTION

Aspects of the present invention describe a system and method forreducing condensable material in a gas product stream. As will bedescribed, a multi-variable predictive controller is utilized to achievebetter separation of oil and gas material. In particular, themulti-variable predictive controller utilizes Reid Vapor Pressure (RVP)identification of the butane molecules in the Dry Oil Tank of theoffshore facility and controls two or more manipulated variables toensure each controlled variable is tightly controlled to an economicallyadvantageous point.

FIG. 1 is a schematic of system 100 used for reducing condensablematerial in a gas product stream. The hydrocarbon product stream 101 isinput into dry oil tank 103. The temperature and pressure within the dryoil tank are respectively monitored via temperature sensor 105 andpressure sensor 107. Temperature sensor 105 and pressure sensor 107 canbe positioned within dry oil tank 103, or in close proximity to dry oiltank 103, such that temperature sensor 105 and pressure sensor 107 canaccurately measure the temperature and pressure conditions (i.e.,measure within specifications set by the International Organization forStandardization (ISO) for temperature and pressure sensors) within dryoil tank 103. In some embodiments, temperature and pressure are measuredby a single unit (i.e., both temperature and pressure are measured bysensor 107).

The measured temperature and pressure associated with dry oil tank 103are sent to indicator 109, which computes the Reid Vapor Pressure fordry oil tank 103. In embodiments, the Reid Vapor Pressure is computed byindicator 109 with respect to the thermodynamics of butane, which allowsoperations to know what amount of butane, and heavier molecules, are inthe oil and what amount are in the gas. For example, Reid Vapor Pressurecan be computed according to the following equations:

RVP=10^([A−B/(100+C)])+Bias

A=Log P ₁ +B/(T₁ +C)

B=(Log P ₁−Log P _(ref))/[1/(T _(ref) +C)−1/(T ₁ +C)]

Indicator 109 communicates the calculated Reid Vapor Pressure tomulti-variable predictive controller 111. While indicator 109 andmulti-variable predictive controller 111 are illustrated in FIG. 1 astwo separate components, one skilled in the art will appreciate thatReid Vapor Pressure calculations can alternatively be computed directlyby multi-variable predictive controller 111. In other embodiments, bothindicator 109 and multi-variable predictive controller 111 compute ReidVapor Pressure calculations. The multi-variable predictive controller111 operates both the temperature and pressure control associated withthe dry oil tank 103 to maintain a desired Reid Vapor Pressurespecification. In embodiments, both the temperature and pressureassociated with the dry oil tank 103 are controlled simultaneously. Inembodiments, the RVP within dry oil tank 103 is maintained within 10% ofthe target RVP. In embodiments, the RVP within dry oil tank 103 ismaintained within 5% of the target RVP.

In embodiments, a multi-variable predictive controller 111, such asAspenTech DMCplus®, is used to collect empirical process information bymeans of a design of experiment (DOE), also referred to as a “step test”in control engineering, on an offshore oil and gas production facility.This empirical data can then be used to build a dynamic process model ofthe facility's behavior using the multi-variable predictive controller111. The dynamic process model can then be commissioned to operateinside the facility's process control network. As previously described,the RVP calculation continuously reads the dry oil tank pressure andtemperature. A move plan is calculated for the dry oil tank outletpressure and input temperature and predictions of RVP values aregenerated based on the facility's current state. Using the RVPpredictions and the economic value of making the best changes, themulti-variable predictive controller carries out set point changes tothe aforementioned temperature and pressure controllers to keep the RVPunder tight, economic control.

The dry oil tank pressure is controlled by adjusting valve 113, whichcontrols the flow of gas effluent out of dry oil tank 103. Inparticular, multi-variable predictive controller 111 communicates acontrol signal to pressure controller 115 to adjust the position (e.g.,open, close, or position somewhere in between) of valve 113 to increaseor decrease the flow of gas effluent out of dry oil tank 103 based onthe calculated RVP by indicator 109. The position of valve 113 isadjusted in real-time to maintain the pressure in the dry oil tank 103within a desired range of the target RVP. While valve 113 and pressurecontroller 115 are illustrated in FIG. 1 as two separate components, oneskilled in the art will appreciate that valve 113 and temperaturecontroller 115 can be combined into a single assembly.

The dry oil tank temperature is controlled by adjusting valve 117, whichcontrols the flow of temperature of hydrocarbon product stream 101entering the dry oil tank 103. In particular, multi-variable predictivecontroller 111 communicates a control signal to temperature controller119 to adjust the position (e.g., open, close, or position somewhere inbetween) of valve 117 to increase or decrease the flow of fluid flowingthrough heat exchanger 121, thereby exchanging heat with hydrocarbonproduct stream 101. In embodiments, heat exchanger 121 can act as acooler or chiller to hydrocarbon product stream 101 (i.e., reduce thetemperature). For example, if the temperature of hydrocarbon productstream 101 is above the desired temperature (based on the calculated RVPby indicator 109) and heat exchanger 121 is a cooler to hydrocarbonproduct stream 101, multi-variable predictive controller 111 cancommunicate a control signal to temperature controller 119 to furtheropen valve 117, thereby increasing the flow of fluid through heatexchanger 121 to reduce the temperature of hydrocarbon product stream101 entering dry oil tank 103. Likewise, if the temperature ofhydrocarbon product stream 101 is below the desired temperature (basedon the calculated RVP by indicator 109), multi-variable predictivecontroller 111 can communicate a control signal to temperaturecontroller 119 to further close valve 117, thereby reducing the flow ofcoolant through cooler 121 to increase the temperature of hydrocarbonproduct stream 101 entering the dry oil tank 103. In other embodiments,heat exchanger 121 can act as a heater to hydrocarbon product stream 101(i.e., increase the temperature). In this case, valve 117 is opened andclosed in the opposite manner to if heat exchanger 121 acts as a cooleror chiller to hydrocarbon product stream 101. Therefore, the position ofvalve 117 is adjusted in real-time to maintain the temperature in thedry oil tank 103 within a desired range of the target RVP. While valve117 and temperature controller 119 are illustrated in FIG. 1 as twoseparate components, one skilled in the art will appreciate that valve117 and temperature controller 119 can be combined into a singleassembly.

System 100 can be implemented on any upstream facility where bothtemperature and pressure of a separation vessel can be controlledsimultaneously. A predictable, well controlled, RVP parameter willprovide a very detailed separation at the molecular level ofhydrocarbons for correct placement in either the gas export stream(through gas effluent line 123 out of dry oil tank 103) or oil exportstream (through oil effluent line 125 out of dry oil tank 103).

FIG. 2 is a schematic of system 200 used for reducing condensablematerial in a gas product stream. The hydrocarbon product stream 201 isinput into dry oil tank 203. The pressure within the dry oil tank ismonitored via pressure sensor 207. The temperature within the dry oiltank is monitored indirectly by monitoring the temperature of a fluid incommunication with hydrocarbon product stream 201. The measuredtemperature and pressure associated with dry oil tank 103 are sent toindicator 209, which computes the Reid Vapor Pressure (similar to system100) for dry oil tank 203. Indicator 209 communicates the calculatedReid Vapor Pressure to multi-variable predictive controller 211. Again,computation of Reid Vapor Pressure could alternatively be computeddirectly by multi-variable predictive controller 211. The multi-variablepredictive controller 211 operates both the temperature and pressurecontrol associated with the dry oil tank 203 to maintain a desired ReidVapor Pressure specification. In embodiments, both the temperature andpressure associated with the dry oil tank 203 are controlledsimultaneously.

The dry oil tank pressure is controlled by adjusting valve 213, whichcontrols the flow of gas effluent out of dry oil tank 203. Inparticular, multi-variable predictive controller 211 communicates acontrol signal to pressure controller 215 to adjust the position (e.g.,open, close, or position somewhere in between) of valve 213 to increaseor decrease the flow of gas effluent out of dry oil tank 203 based onthe calculated RVP by indicator 209. The position of valve 213 isadjusted in real-time to maintain the pressure in the dry oil tank 203within a desired range of the target RVP. In the embodiment shown inFIG. 2, gas stream flows through gas effluent line 229 out of dry oiltank 203 to vapor recovery unit 216 where the gas stream is furtherseparated. Gas is fed into a compressor 217 where it is compressed anddelivered to a sales gas pipeline. Any condensate from vapor recoveryunit 216 can be delivered to a sales oil pipeline (e.g., oil exportstream through oil effluent line 231), back into hydrocarbon productstream 201, or into other processing equipment. Liquid level sensor 219can monitor the level of liquid within vapor recovery unit 216 and openvalve 221 as necessary to purge vapor recovery unit 216 of condensate.

The dry oil tank temperature is controlled by adjusting valve 223, whichcontrols the flow of temperature of hydrocarbon product stream 201entering the dry oil tank 203. In particular, multi-variable predictivecontroller 211 communicates a control signal to temperature controller225 to adjust the position (e.g., open, close, or position somewhere inbetween) of valve 223 to increase or decrease the flow of fluid flowingthrough heat exchanger 227, thereby exchanging heat with hydrocarbonproduct stream 201. In embodiments, heat exchanger 227 can act as acooler or chiller to hydrocarbon product stream 201 (i.e., reduce thetemperature) or act as a heater to hydrocarbon product stream 201 (i.e.,increase the temperature). In this embodiment, the temperature of thefluid communicating with the hydrocarbon product stream 201 in heatexchanger 227 is monitored downstream of heat exchanger 227. Based onany temperature change to this fluid, multi-variable predictivecontroller 211 adjusts valve 223 to increase or decrease heat exchangefrom hydrocarbon product stream 201, thereby adjusting the temperatureof the hydrocarbon product stream 201 entering the dry oil tank 203.

System 200 can be implemented on any upstream facility where bothtemperature and pressure of a separation vessel can be controlledsimultaneously. A predictable, well controlled, RVP parameter willprovide a very detailed separation at the molecular level ofhydrocarbons for correct placement in either the gas export stream(through gas effluent line 229 out of dry oil tank 203) or oil exportstream (through oil effluent line 231 out of dry oil tank 203).

The system illustrated in FIG. 2 was tested on a deepwater oilproduction platform in the Gulf of Mexico. FIG. 3 illustrates a graph ofthe resultant RVP improvement. Using univariate regulatory control 305,which only controls pressure and temperature independently of each otherby the human operator, resulted in a mean value of 5.48 PSIG with astandard deviation of 0.13 PSIG. Using the multi-variable predictivecontroller 310 resulted in a significantly higher mean value of 9.64PSIG with a standard deviation of 0.02 PSIG. Accordingly, themulti-variable predictive controller optimized the control process,thereby keeping the RVP under tight, economic control.

As used in this specification and the following claims, the terms“comprise” (as well as forms, derivatives, or variations thereof, suchas “comprising” and “comprises”) and “include” (as well as forms,derivatives, or variations thereof, such as “including” and “includes”)are inclusive (i.e., open-ended) and do not exclude additional elementsor steps. Accordingly, these terms are intended to not only cover therecited element(s) or step(s), but may also include other elements orsteps not expressly recited. Furthermore, as used herein, the use of theterms “a” or “an” when used in conjunction with an element may mean“one,” but it is also consistent with the meaning of “one or more,” “atleast one,” and “one or more than one.” Therefore, an element precededby “a” or “an” does not, without more constraints, preclude theexistence of additional identical elements.

The use of the term “about” applies to all numeric values, whether ornot explicitly indicated. This term generally refers to a range ofnumbers that one of ordinary skill in the art would consider as areasonable amount of deviation to the recited numeric values (i.e.,having the equivalent function or result). For example, this term can beconstrued as including a deviation of ±10 percent of the given numericvalue provided such a deviation does not alter the end function orresult of the value. Therefore, a value of about 1% can be construed tobe a range from 0.9% to 1.1%.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for the purpose of illustration, it will be apparentto those skilled in the art that the invention is susceptible toalteration and that certain other details described herein can varyconsiderably without departing from the basic principles of theinvention. For example, the invention can be implemented in numerousways, including for example as a method (including acomputer-implemented method), a system (including a computer processingsystem), an apparatus, a computer readable medium, a computer programproduct, a graphical user interface, a web portal, or a data structuretangibly fixed in a computer readable memory.

What is claimed is:
 1. A method for separating a multi-phase fluid on anoffshore platform, the method comprising: (a) providing a separationvessel containing a multi-phase fluid; (b) measuring a pressure and atemperature associated with the separation vessel; (c) calculating aReid Vapor Pressure for the separation vessel based on the pressure andthe temperature associated with the separation vessel; and (d) activelycontrolling the pressure and the temperature associated with theseparation vessel such that the calculated Reid Vapor Pressure for theseparation vessel is maintained within a predetermined amount of areference Reid Vapor Pressure.
 2. The method of claim 1, wherein thepressure and the temperature associated with the separation vessel areadjusted simultaneously based on the calculated Reid Vapor Pressure. 3.The method of claim 1, wherein the calculated Reid Vapor Pressure forthe separation vessel is maintained within ten percent of the referenceReid Vapor Pressure.
 4. The method of claim 1, wherein the calculatedReid Vapor Pressure for the separation vessel is maintained within fivepercent of the reference Reid Vapor Pressure.
 5. The method of claim 1,wherein the temperature associated with the separation vessel isactively controlled by a heat exchanger upstream of the separationvessel.
 6. The method of claim 1, wherein the Reid Vapor Pressure iscalculated according to the following equations:RVP=10^([A−B/(100+C)])+BiasA=Log P ₁ +B/(T₁ +C)B=(Log P ₁−Log P _(ref))/[1/(T _(ref) +C)−1/(T ₁ +C)].
 7. The method ofclaim 1, wherein the Reid Vapor Pressure is calculated according to thethermodynamics of butane.
 8. The method of claim 1, further comprisinggenerating an empirical dynamic process model of the offshore platformand utilizing the empirical dynamic process model to determineadjustments made to the pressure and the temperature associated with theseparation vessel.
 9. A system for separating a multi-phase fluid on anoffshore platform, the system comprising: (a) a separation vessel thatreceives a multi-phase fluid; (b) a pressure sensor that monitorspressure associated with the separation vessel; (c) a temperature sensorthat monitors temperature associated with the separation vessel; and (d)a multi-variable predictive controller that actively controls thepressure and the temperature associated with the separation vessel suchthat a calculated Reid Vapor Pressure for the separation vessel ismaintained within a predetermined amount of a reference Reid VaporPressure.
 10. The system of claim 9, further comprising an indicator tocalculate the Reid Vapor Pressure for the separation vessel based on thepressure monitored by the pressure sensor and the temperature monitoredby the temperature sensor.
 11. The system of claim 9, wherein themulti-variable predictive controller maintains the calculated Reid VaporPressure for the separation vessel within ten percent of the referenceReid Vapor Pressure.
 12. The system of claim 9, wherein themulti-variable predictive controller maintains the calculated Reid VaporPressure for the separation vessel within five percent of the referenceReid Vapor Pressure.
 13. The system of claim 9, wherein themulti-variable predictive controller further calculates the Reid VaporPressure for the separation vessel based on the pressure monitored bythe pressure sensor and the temperature monitored by the temperaturesensor.
 14. The system of claim 9, wherein the Reid Vapor Pressure iscalculated according to the following equations:RVP=10^([A−B/(100+C)])+BiasA=Log P ₁ +B/(T₁ +C)B=(Log P ₁−Log P _(ref))/[1/(T _(ref) +C)−1/(T ₁ +C)]
 15. The system ofclaim 9, wherein the Reid Vapor Pressure is calculated according to thethermodynamics of butane.
 16. The system of claim 9, wherein thepressure and the temperature associated with the separation vessel areadjusted simultaneously based on the calculated Reid Vapor Pressure. 17.The system of claim 9, further comprising a heat exchanger upstream ofthe separation vessel that is used to adjust the temperature associatedwith the separation vessel.
 18. The system of claim 9, wherein thepressure and the temperature associated with a separation vessel areadjusted simultaneously based on the calculated Reid Vapor Pressure. 19.The system of claim 9, wherein the multi-variable predictive controlleris further used to generate an empirical dynamic process model of theoffshore platform to determine adjustments made to the pressure and thetemperature associated with the separation vessel.